This invention relates to transistors and, more particularly, to bipolar transistors having improved reversed second breakdown capabilities.
Reverse second breakdown can occur while the transistor is being switched off. During switch off, the majority carriers in the base flow laterally from beneath the emitter to the base electrode causing a lateral voltage drop that turns off the outer edge portions of the emitter, but leaves the central portion thereof on. That is, the central portion of the emitter still injects minority carriers. Any stray or load inductance in the circuit connected to the collector forces the collector current to remain constant so that the central portion of the emitter injects more and more minority carriers. Accordingly, the current density under the central portion of the emitter increases to provide the constant current. This high current density causes base widening in the central region under the emitter which, in turn, changes the electric field distribution so that the peak field increases and is located at the N+N interface of the collector. If the peak field becomes sufficiently high, the current and voltage experience avalanche multiplication causing regeneration, excessive heating and finally catastrophic destruction of the transistor.
One attempt to increase the reverse second breakdown capability of transistors is disclosed in British Patent Application No. 2,026,236A published Jan. 30, 1980. In accordance with this publication, an attempt is made to decrease the current density under the central portion of the emitter when the transistor is being switched off. This is accomplished by forming the emitter with a central portion having a lower gain than the outer edge portions thereof. Various techniques are disclosed for providing such an emitter. The problem with this approach is that considerable emitter injecting area is lost which leads to a loss in the saturation voltage and maximum current handling capability of the transistor.